The Chromosomal translocation (3;21)(q26;q22) is a recurring abnormality in the malignant cells of patients with chronic myelogenous leukemia in blast crisis and with acute myeloid leukemia primarily following treatment with topoisomerase II inhibitors. The gene that is involved at 21q22 is AML1.AML1 encodes the DNA-binding alpha subunit of the core transcription factor CBF, and has homology to the murine transcription factor Pebp2alpha and to the D. melanogaster segmentation gene runt.AML1 is involved in several chromosomal translocations. In the 8;21 translocation in acute myeloid leukemia, AML1 fuses to ETO at 8q22. ETO also is a transcription factor based on proline-rich regions and two zinc finger motifs. In the t(3;21, AML1 fuses to EAP which encodes the abundant ribosomal protein L22 at 3q26. Transcription starts at the AML1 promoter, but includes at least one other gene downstream of EAP, and the nuclear transcript undergoes a unique and complex splicing pattern resulting in multiple fusion transcripts of AML1 with two unrelated genes of chromosome 3. In five t(3;21) patients whom we studied, we consistently detected the AML1/EAP fusion transcript, and, in addition, a second chimeric transcript that fuses AML1 to sequences not previously reported. We have called this locus MDS1 for myelodysplasis syndrome. In one of the five patients, additional fusion transcripts have been identified that fuse AML1 to MDS1 and to EVI1, or AML1. EVI1 is a well characterized transcription factor and is 400-700 kb downstream of EAP. Our current data suggest that MDS1 is a new gene, although we cannot exclude that it may represent undetected exons of EVI1. The underlying hypothesis regarding the role of fusion genes resulting from chromosomal translocations in leukemogenesis is that the proteins whose properties are modified by the fusion are directly involved in the development of progression of the leukemia. However, the t(3;21) is unique in that this is the first example in which at least two fusion genes are consistently produced by the translocation, and this raises the question of which of the altered genes is responsible for the disease (or perhaps a set of the chimeric genes is necessary to the transformation). The overall aim of this project is to characterize in detail the normal genes on 3q26 and the new fusion genes that are consistently expressed from the same translocated region of AML1, with emphasis on their role in leukemogenesis. Such as analysis involves determining 1) whether MDS1 is a new gene or part of EVI1, 2) the location and function of its gene product in the cell, 3) the alterations in the cellular location and functions of the fusion proteins, and 4) the manner in which the altered protein(2) contribute to the development of clinical progression of leukemia.